The present invention relates to a solid state image sensing apparatus and a method for driving the apparatus, more particularly to a solid state image sensing apparatus for use by an electronic still camera and a method for driving the apparatus incorporated in the camera. The invention further relates to cameras such as electronic still cameras comprising a solid state image sensing apparatus each.
Along with the development in recent years of CCD (charge coupled devices) for image sensing and other solid state image sensing apparatuses has come widespread use of electronic still cameras. These cameras record images digitally using CCD-equipped solid state image sensing apparatuses as their image sensors.
To achieve high levels of resolution, electronic still cameras usually employ a so-called all-pixel read-out type CCD-equipped solid state image sensing apparatus. In operation, signal charges of the pixels represented by each vertical CCD column are read out independently of one another at the same time, as shown in FIG. 9. Given the same number of pixels, the all-pixel read-out type CCD-equipped solid state image sensing apparatus doubles the vertical resolution of the so-called field read-out type solid state image sensing apparatus used generally by video cameras.
There has recently been a demand for reducing power dissipation of these electronic still cameras so that they may be operated for an extended period of time. One way of achieving that target is to reduce power dissipation by the solid state image sensing apparatus of the electronic still camera, especially by a solid state image sensing device constituting the solid state image sensing apparatus and by a signal processing circuit for processing signals from these components.
Power consumption of a solid state image sensing device depends significantly on their driving frequency. This signifies that lowering the driving frequency of the solid state image sensing device effectively reduces the amount of power consumed thereby. Since the driving frequency is also used as the frequency of the signal processing circuit, reductions in the driving frequency are expected to reduce the amount of power dissipation by the signal processing circuit as well.
There is a disadvantage in lowering the driving frequency of the solid state image sensing device: it also lowers their frame rate. A lowered frame rate reduces the amount of output signals (i.e., number of pictures) output by the solid state image sensing device per unit time. This translates into awkward and unnatural motions of pictures shown on a display unit (typically an LCD) of the electronic still camera for the purpose of verifying the composition of a subject. Another disadvantage is increased vulnerability of the solid state image sensing device to adverse effects of dark currents and smears.